1. Field of the Invention
The present invention relates to a rotational angle sensing device, which measures a relative rotational angle between two members (e.g., a rotatable member and a non-rotatable member) without making any physical contact. The present invention further relates to an assembling method of such a rotational angle sensing device.
2. Description of Related Art
FIGS. 10A and 10B show a prior art rotational angle sensing device, which measures a wide range of rotational angles through use of a magnet and magnetic sensing elements (see for example, Japanese Unexamined Patent Publication No. 2003-75108).
The rotational angle sensing device includes a circular disk shaped magnet 2 and first and second Hall ICs 3, 4. An outer peripheral circular edge of the magnet 2 is substantially coaxial with a rotatable shaft 1. The magnet 2 has a magnetic flux generating portion (N pole) and a magnetic flux attracting portion (S pole), which are diametrically opposed to each other in a plane perpendicular to the rotatable shaft 1. In FIG. 10B, the first and second Hall ICs 3, 4 are arranged below the outer peripheral edge of the magnet 2 in such a manner that the second Hall IC 4 is displaced from the first Hall IC 3 by 90 degrees in a rotational direction of the magnet 2. The first and second Hall ICs 3, 4 output signals, which correspond to the magnetic flux outputted from the magnet 2.
When the magnet 2 is rotated, the first and second Hall ICs 3, 4 generate a sine wave output signal and a cosine wave output signal, respectively, like ones shown in FIG. 5A. An angle computing circuit (a microcomputer) converts the sine wave signal and the cosine wave signal through use of an inverse trigonometric function into a characteristic saw tooth waveform, which is similar to one shown in FIG. 5B and is periodically repeated at 180 degree intervals. The characteristic saw tooth waveforms are connected one after another to obtain a rotational angle output ranging from 0 degree to 360 degrees, like one shown in FIG. 5C.
In the above prior art rotational angle sensing device, the first Hall IC 3 and the second Hall IC 4 are arranged separately from one another. Therefore, a space for accommodating the first Hall IC 3 and a space for accommodating the second Hall IC 4 are respectively required. Therefore, in a case where the available accommodating space is relatively small, it is difficult to provide the accommodating spaces for accommodating the first and second Hall ICs 3, 4. Thus, in such a case, when the prior art rotational angle sensing device is used, the first Hall IC 3 and the second Hall IC 4 cannot be spaced from one another, and therefore the rotational angle cannot be measured. Furthermore, in the prior art rotational angle sensing device, since the first Hall IC 3 and the second Hall IC 4 are spaced from one another, a difference in the environmental condition (e.g., temperature) may exist between the first Hall IC 3 and the second Hall IC 4 to cause a substantial difference in the output between the first Hall IC 3 and the second Hall IC 4, resulting in deterioration of the accuracy of the measured angle.